An implantable pacer can be used for pacing a heart. An example of pacing can include bradycardia pacing, which can deliver an electrostimulation pulse to the heart to evoke a responsive heart contraction, such as to maintain a fast enough heart rate to provide a cardiac output of blood to meet a patient's metabolic need. Another example of pacing can also include antitachyarrhythmia pacing (ATP), which can include delivering a quick sequence of electrostimulations, such as to “overdrive” a too-fast tachyarrhythmic heart rhythm so that the ATP pulses take control of the heart rhythm; then the ATP pulse rate can be lowered to an appropriate heart rate.
An implantable cardiac resynchronization therapy (CRT) device can be used for spatially coordinating heart contractions. CRT can include delivering electrostimulations to maintain one or more of atrioventricular (AV) timing, interatrial timing (LA-RA) timing, interventricular timing (LV-RV), intraventricular timing, or the like.
An implantable defibrillator can be used for delivering a higher-energy cardioversion shock to interrupt an abnormal heart rhythm, such as an atrial or ventricular tachyarrhythmia or fibrillation.
A cardiac contractility modulation (CCM) device can be used for delivering an non-stimulatory energy to the heart to increase heart contractility (since a stronger heart contraction can also help increase cardiac output, along with a higher contraction rate, and proper AV or other synchrony) rather than to increase the heart rate (like pacing) or to spatially synchronize a heart contraction (like CRT). In CCM therapy, electrical energy is typically delivered to the heart during a refractory period of the heart, such as a time immediately following a heart contraction. During a refractory period, the heart tissue is insensitive to electrostimulation in that electrostimulations delivered during the refractory period do not evoke a resulting heart contraction. However, the CCM electrical energy delivered during the refractory period, although it does not evoke a responsive heart contraction, is believed to be capable of increasing heart contractility, such that the next heart contraction can be more forceful, which should help yield better cardiac output.
There are two refractory periods associated with ventricular cardiac tissue, an absolute refractory period and a relative refractory period. The absolute ventricular refractory period begins at the start of the action potential and includes the QRS complex and the positive going portion of the T wave. The relative refractory period occurs during the negative going portion of the T wave. During the absolute ventricular refractory period ventricular tissue cannot be stimulated to begin another action potential (or resulting contraction). During the relative ventricular refractory period, ventricular tissue can be stimulated to begin another action potential (and the resulting contraction), however a larger stimulus than normal is typically required. Further, delivery of electrical energy during the relative refractory period can be proarrhythmic.
Since is it intended that CCM therapy directed at proarrhythmic tissue not trigger ventricular contractions, the electrical energy associated with proarrhythmic CCM therapy is preferably delivered during the absolute ventricular refractory period. Further the energy associated with CCM therapy directed at atrial tissue would be delivered during the absolute atrial refectory period.